In extant primate brains, this is manifest by the position of the lunate sulcus, the anterior boundary of the primary visual striate cortex.
Holloway, "The Past, Present, and Future Significance of the Lunate Sulcus in Early Hominid Evolution," in Hominid Evolution: Past, Present, and Future, ed.
Holloway, "The Past, Present, and Future Significance of the Lunate Sulcus in Early Hominid Evolution," in Brain Endocasts--The Paleoneurological Evidence, ed.